Beta-type titanium alloy having low elastic modulus and high strength

ABSTRACT

Provided is a beta-type titanium alloy having a low elastic modulus and a high strength. The titanium alloy includes 6 to 13 wt % of Mo, 0.1 to 3.9 wt % of Fe, a remaining amount of Ti, and inevitable impurity, and selectively includes 0.1 to 3.9 wt % of Al. The titanium alloy according to the present invention has a high tensile strength of greater than or equal to 1,300 MPa and a low elastic modulus of less than or equal to 95 GPa at low cost.

TECHNICAL FIELD

The present invention relates to a beta-type titanium alloy having a low elastic modulus and a high strength and more particularly, to a beta-type titanium alloy having a low elastic modulus and a high strength, and having a high performance at low cost.

BACKGROUND ART

Titanium has a high specific strength (strength/weight) and a good corrosion-resistance and has a high applicability as a base material in various industrial fields. Thus, titanium is called as an advanced material of a dream and is one of advanced metal materials expected to be used in a future application. Due to the various good properties of the titanium, researches in biomedical, marine, aerospace, sports and leisure fields have been widely conducted.

A titanium alloy is generally classified into a α-type (hexagonal crystal close-packed: hcp), a β-type (body-centered cubic: bcc), and a α+β-type based on the crystalline structure of the phase constituting a metal structure at room temperature. An alloy obtained by adding a small amount of such as aluminum or titanium for industry is the α-type. Ti-6Al-4V alloy known as a high strength alloy and used in an airplane is the α+β-type, and the β-type is an alloy including an increased amount of an alloy element for stabilizing a β-phase.

When comparing with a steel material, the titanium alloy has a density of about 56% and a shear elastic modulus of about 50% with respect to those of the steel material at the states having the same strength. Thus, a spring having the same performance may have a theoretical weight of about 28%, and the lightening by about 72% with respect to the steel material may be attainable.

When manufacturing a coil spring by using Ti-3Al-8V-6Cr-4Mo-4Zr (β-C) alloy, the spring may be useful to the maximum shear stress of 839 MPa. In addition, the weight of the coil spring is only about 47% of the coil spring manufactured by using a steel material having the same performance, and a lightening effect may be certainly attainable.

In addition, since the titanium alloy has high damping capacity and natural frequency, a surging phenomenon constituting a problem during the high-speed rotation of an engine may be evitable, and the extension of a lifetime may be accomplished. The natural frequency of the Ti-3Al-8V-6Cr-4Mo-4Zr alloy, which is commonly used as a spring, is 870 Hz, which is superior to that of the spring manufactured by the common steel material, 483 Hz.

In addition, since the shear elastic modulus of the titanium alloy spring material is small and about 50% of the common spring steel, the number of the winding of the spring may be decreased. In addition, the miniaturization and lightening of the engine may be promoted by decreasing the contacting height of a valve spring. Further, due to the various properties of the titanium alloy as described above, when the spring of the titanium alloy is used for a suspension in vehicles, a good cushioning effect may be obtained to improve a ride comfort.

However, when considering the weight of the vehicles, the tensile strength of the titanium alloy for the suspension spring is at least 1,300 MPa or over. In order to sufficiently obtain the titanium effect as described above, the elastic modulus of the titanium alloy is preferably less than or equal to 95 GPa.

Meanwhile, in order to manufacture the p-type titanium alloy, a quite amount of beta stabilizing elements is required to be included. Since the beta stabilizing elements are generally expensive, the usage thereof is limited to special parts requiring good physical properties as described above.

Even though the titanium alloy has good properties, since low-priced parts are used in vehicles, the titanium alloy may not be replaced with the common steel material for manufacturing the parts.

DISCLOSURE OF THE INVENTION Technical Problem

The purpose of the present invention is to solve the above-described defects and to provide a titanium alloy having good physical properties such as a high tensile strength of greater than or equal to 1,300 MPa and a low elastic modulus of less than or equal to 95 GPa at low cost.

Technical Solution

There is provided to solve the above defects in the present invention a beta-type titanium alloy having a low elastic modulus and a high strength, including 6 to 13 wt % of Mo, 0.1 to 3.9 wt % of Fe, a remaining amount of Ti, and inevitable impurity. The titanium alloy has a tensile strength of greater than or equal to 1,300 MPa and an elastic modulus of less than or equal to 95 GPa.

In addition, according to an aspect of the present invention, the titanium alloy may further include 0.1 to 3.9 wt % of Al. That is, there is provided in the present invention a beta-type titanium alloy having a low elastic modulus and a high strength, including 6 to 13 wt % of Mo, 0.1 to 3.9 wt % of Fe, 0.1 to 3.9 wt % of Al, a remaining amount of Ti, and inevitable impurity, wherein the titanium alloy has a tensile strength of greater than or equal to 1,300 MPa and an elastic modulus of less than or equal to 95 GPa. The addition of Al may increase the processability, the formability, the castability, etc. and may provide advantages for applying various heat treatment techniques to obtain reinforcing effects.

In addition, according to an aspect of the present invention, 0.005 to 0.5 wt % of B may be additionally included in the titanium alloy.

In addition, according to an aspect of the present invention, an elongation percentage of the titanium alloy may be greater than or equal to 6%.

In addition, according to an aspect of the present invention, the tensile strength of the titanium alloy may be greater than or equal to 1,400 MPa.

In addition, according to an aspect of the present invention, the microstructure of the titanium alloy may include omega (ω) phase particles minutely dispersed in a beta (β) matrix. The omega (ω) phase may be removed or generated by using a heat treatment technique to obtain required strength, ductility and elastic modulus.

Advantageous Effects

The titanium alloy according to the present invention has a tensile strength of greater than or equal to 1,300 MPa and an elastic modulus of less than or equal to 95 GPa, and may be applied in various fields requiring a low elastic modulus and a high strength.

In addition, since the use of an expensive alloy element in the titanium alloy is minimized according to the present invention, the manufacturing cost of the alloy may be largely decreased.

In addition, when the titanium alloy is manufactured through forging and rolling, a rod material, a rectangular lumber and a plate material having a tensile strength of greater than or equal to 1,300 MPa, an elastic modulus of less than or equal to 95 GPa, and an elongation percentage of greater than or equal to about 6% may be manufactured, without conducting a heat treatment such as a solution heat treatment or an ageing. Thus, a spring for transportation vehicles and parts having a high strength and a low elastic modulus in various fields may be manufactured at low cost. Particularly, when the titanium alloy is used for a spring material, lightening to about 50 to 60% when compared with a spring manufactured by using a steel material may be accomplished.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 are photographs on microstructures of hot forged materials respectively manufactured in Examples 1 to 5 of the present invention and taken by an optical microscope.

FIGS. 6 and 7 are photographs on microstructures of rolled rod materials manufactured in Examples 2 and 3 of the present invention and taken by an optical microscope.

FIGS. 8 and 9 are photographs on microstructures of rolled rod materials respectively manufactured in Examples 2 and 3 and taken by a dark field image (DFI) transmission electron microscope.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described in detail, however, the present invention is not limited to the following embodiments.

First, the composition ranges of each alloy element in the beta-type titanium alloy according to the present invention are defined as follows.

Mo: 6 to 13 wt %

Mo is a stabilizing element of a beta (β) phase and has an effect of lowering an elastic modulus and increasing strength. Since Mo is expensive, the amount of Mo is optimized to lower the cost while obtaining mechanical properties. Preferable amount of Mo is 6 to 13 wt %.

Fe: 0.1 to 3.9 wt %

Fe is a stabilizing element of a beta (β) phase but increases a deformation resistance. Thus, Fe has been added as small as possible in the prior arts. In the present invention, relatively a large amount of cheap Fe is used when compared with other beta stabilizing elements. When the amount of Fe is less than 0.1 wt %, the beta stabilizing effect is insufficient, and when the amount of Fe exceeds 3.9 wt %, the deformation resistance may be excessive to deteriorate processing properties. Thus, the preferred amount of Fe is less than or equal to 3.9 wt %.

Meanwhile, the index on manufacturing the beta-type titanium alloy having a low elastic modulus by stabilizing the beta (β) phase may be illustrated by Mo equivalent in [Equation 1]. The preferred Mo equivalent when calculated with Fe is about 7.0 to 20.0.

Mo equivalent=[Mo]+1/5[Ta]+1/3.6[Nb]+1/2.5[W]+1/1.5[V]+1.25[Cr]+1.25[Ni]+1.7[Mn]+1.7[Co]+2.5[Fe]  [Equation 1]

Al: 0.1 to 3.9 wt %

Al is an element added to improve the strength of the β-type titanium alloy according to the present invention. Al restrains the precipitation of an omega (ω) phase which increases the hardness of the titanium alloy by embrittlement during heat treatment, increases the strength and the ductility, and improves processability and castability. Al may be selectively added in the present invention. When the amount of Al exceeds 3.9 wt %, the hardness may be excessively increased, and the elongation percentage may be lowered to decrease the processability. Thus, the amount of Al added is preferably less than or equal to 3.9 wt %.

B: 0.005 to 0.5 wt %

B is an element restraining the growth of a huge solidification structure while conducting solution cast. When B is added by less than 0.005 wt %, the enlargement of the solidification structure may be ineffectively restrained. When the amount of B exceeds 0.5 wt %, further miniaturization of the cast structure may not be accomplished. Thus, the preferred amount of B is 0.005 to 0.5 wt %.

Inevitable Impurities

The inevitable impurities are components possibly added in the raw material of the titanium alloy or during processing unintentionally. Particularly, oxygen may deteriorate the deformation capacity of the titanium alloy, may become a reason generating cracks during cold working, and may become a reason increasing a deformation resistance. Thus, the amount of the inevitable impurities is required to be maintained by less than or equal to 0.3 wt % and is preferably required to be less than or equal to 0.18 wt %. In addition, since hydrogen deteriorates the ductility and the toughness of the titanium alloy, the amount of the hydrogen is preferably as small as possible. The amount of the hydrogen is more preferably, less than or equal to 0.03 wt %, and most preferably, less than or equal to 0.01 wt %. Carbon largely lowers the deformation capacity of the titanium alloy and so, is required to be included as small amount as possible. Preferably, the amount of the carbon is less than or equal to 0.05 wt % and more preferably, the amount of the carbon is less than or equal to 0.01 wt %. In addition, nitrogen also largely lowers the deformation capacity of the titanium alloy and so is required to be included as small amount as possible. Preferably, the amount of the nitrogen is less than or equal to 0.02 wt % and more preferably, the amount of the nitrogen is less than or equal to 0.01 wt %.

In addition, in the microstructure of the titanium alloy according to the present invention, an alpha (α) phase may be mixed in a beta phase matrix, and minutely dispersed omega (ω) phase particles may be included in the beta (β) phase matrix.

A method of processing a rod material, a rectangular lumber and a plate material by using a titanium alloy according to the present invention includes (a) preparing a titanium alloy liquid metal including the above described components; (b) casting the thus prepared titanium alloy to manufacture an ingot; (c) hot forging the ingot at 800° C. to 1,200° C.; and (d) rolling the forged titanium alloy at 25° C. to 650° C.

The temperatures of the hot forging and the rolling are preferably maintained within the above defined ranges to prevent the generation of cracks during processing and to obtain a sufficient reduction ratio.

Examples

Titanium alloys having the composition as illustrated in the following Table 1 were manufactured by using induction skull melting (ISM). The amount of the impurities such as oxygen (O), nitrogen (N), carbon (C), and hydrogen (H) in all alloys was less than 0.5 wt %.

TABLE 1 Composition (wt %) Alloy Mo Fe Al Ti Example 1 9.2 2.2 — Bal. Example 2 12.1 1.0 — Bal. Example 3 9.0 2.2 2.0 Bal. Example 4 9.2 2.3 3.1 Bal. Example 5 11.7 1.3 1.2 Bal. Comparative 15.0 — — Bal. example 1 Comparative 3.4 4.0 — Bal. example 2 Comparative 0.5 5.0 — Bal. example 3

The liquid metal of the alloy molten by the composition illustrated in the above Table 1 was cast into an ingot having a size of 100 mm diameter×90 mm height.

In order to select an alloy having sufficient mechanical properties applicable in a suspension of a transportation vehicles among the alloys illustrated in Table 1, the ingots were heated at 1,100° C. by the inventors of the present invention. Then, the ingots were charged into a hot forging machine and were hot forged to manufacture subject materials having a length of 120 mm, a width of 60 mm, and a height of 40 mm.

The tensile properties of the thus hot forged subject materials were evaluated and the results are illustrated in the following Table 2.

TABLE 2 Yield Tensile Elongation Alloy strength (MPa) strength (MPa) percentage (%) Example 1 1198 1204 10 Example 2 769 887 8.5 Example 3 772 895 17 Example 4 809 934 8.0 Example 5 655 806 16 Comparative 896 898 18 example 1 Comparative 1088 1192 5.4 example 2 Comparative 780 916 10 example 3 * Comparative example 1 corresponds to the tensile properties of a common material. * Comparative examples 2 and 3 correspond to the tensile properties measured by using test samples manufactured in rod material shapes.

It may be confirmed that from the evaluation results as illustrated in Table 2, the alloy according to Example 1 of the present invention has the tensile strength exceeding 1,200 MPa and the elongation percentage up to 10%, and illustrates very close properties to the target physical properties of the present invention. Thus, the alloy in Example 1 was confirmed to accomplish the object of the present invention through additional processes.

In addition, even though the alloys in Examples 2 to have the tensile strengths less than 1,000 MPa, the elongation percentages are good and the elastic modulus are less than 95 GPa. Therefore, the strength may be additionally increased through subsequent processes, and the alloys in Examples 2 to 5 may be applied as springs for a suspension of a transfer machine.

In order for confirmation, the alloy in Example 2 and the alloy in Example 3 were respectively selected from Examples 2 and 5, and Examples 3 and 4, which have similar contents of Mo and Fe, respectively. Then, the subsequent processes were conducted and tensile properties and elastic properties were measured.

On the other hand, the alloys according to Comparative examples 1, 2 and 3 may be rod material state after completing subsequent processes, or may have too low tensile properties or too high elastic modulus. Thus, these alloys may be hardly applied as a spring.

Based on the tensile properties at room temperature, the strong candidates of the titanium alloy ingots according to Examples 1, 2 and 3 were forged to manufacture the titanium alloy into rectangular lumbers or rod materials. Then, the titanium alloy was heated to 600° C. and passed three times of rolling to manufacture a rod material having a diameter of 16 to 20 mm and a length of 500 mm or over.

Samples were taken from the center portion of the thus manufactured rod material, and the properties of the samples were evaluated.

First, the microstructure of the rolled rod material was analyzed by using an optical microscope and a transmission electron microscope. The samples for analysis by means of the optical microscope were prepared by a standard metallic preparation process. First, the samples were mirror polished and etched by using an etching solution.

In addition, the samples for analysis by means of the transmission electron microscope were prepared by grinding the samples to a thickness of 60 μm, and conducting a twin-jet polishing in a solution including 35% of butanol-6% of perchloric acid-methanol under 60V condition.

FIGS. 1 to 5 are photographs on microstructures of hot forged materials taken by an optical microscope on the ingots manufactured by hot forging the alloys having the compositions as in Examples 1 to 5. As illustrated in FIGS. 1 to 5, all of the microstructures of the alloys have a β-phase matrix constituted by an equiaxed structure having an average size of about several hundreds μm. In addition, a portion of alpha phase was precipitated and present in the microstructure.

For the photographs in FIGS. 6 and 7, obtained by observing on the microstructures of rolled rod materials by means of the optical microscope, the crystal grains of the microstructures clearly differentiated in the hot forged material disappeared while conducting a subsequent rolling process. In addition, the microstructure was observed to have a wave shape after undergoing a severe plastic deformation. Meanwhile, the microstructures observed through a DFI by using the transmission electron microscope were observed to have minute omega (ω) phase having the size of from several nanometers to several tens nanometers present in some regions.

Then, the tensile properties and the elastic modulus of the rod materials manufactured according to the examples of the present invention and the comparative examples were measured and illustrated in the following Table 3.

TABLE 3 Elonga- Yield Tensile tion Elastic Sam- strength strength percent- modulus ples Composition (MPa) (MPa) age (%) (GPa) Exam- Ti—9.2Mo—2.2Fe 1330 1352 7.0 88 ple 1 Exam- Ti—12.1Mo—1Fe 1440 1501 10.1 85 ple 2 Exam- Ti—9Mo—2.2Fe— 1386 1498 6.9 76 ple 3 2Al Compar- Ti—3.4Mo—4Fe 1088 1192 5.4 93 ative exam- ple 2 Compar- Ti—0.5Mo—5Fe 780 916 10 96 ative exam- ple 3

As confirmed in Table 3, the alloy rod materials according to the examples of the present invention have the tensile strength greater than 1,300 MPa and the elastic modulus less than 95 GPa. Thus, these rod materials satisfied the physical properties required for parts having low elastic modulus and high repulsion properties in various fields including spring materials for a suspension of transportation vehicles.

Particularly, the alloy rod material according to Example 2 has a high tensile strength of 1,501 MPa, a good processability with the elongation percentage of about 10% and a low elastic modulus of 85 GPa. Thus, the alloy rod may be appropriately used as a spring material such as the spring material of a suspension for vehicles.

The alloy rod material according to Example 3 also has a high tensile strength of 1,498 MPa, the elongation percentage of about 7%, and the elastic modulus of 76 GPa, and may be appropriately used in parts requiring a high strength and low elastic properties in various fields.

In addition, the alloy rod material according to the present invention uses cheap iron (Fe) as the beta stabilizing element and confirms good mechanical properties when compared with a common material, Ti-15Mo at low cost. Thus, the manufacturing cost of the titanium alloy may be lowered when compared with the common titanium alloy, and good tensile properties and good elastic properties may be obtained when compared with the common titanium alloy. 

1. A beta-type titanium alloy having a low elastic modulus and a high strength, comprising 6 to 13 wt % of Mo, 0.1 to 3.9 wt % of Fe, a remaining amount of Ti, and inevitable impurity, the titanium alloy having a tensile strength of greater than or equal to 1,300 MPa and an elastic modulus of less than or equal to 95 GPa.
 2. The titanium alloy of claim 1, further comprising 0.1 to 3.9 wt % of Al.
 3. The titanium alloy of claim 1, wherein a microstructure of the beta-type titanium alloy has a dispersed shape of an omega phase having an average particle size of 100 nm or less.
 4. The titanium alloy of claim 1, wherein an Mo equivalent defined by following [Equation 1] is 7.0 to 20.0, Mo equivalent=[Mo]+1/5[Ta]+1/3.6[Nb]+1/2.5[W]+1/1.5[V]+1.25[Cr]+1.25[Ni]+1.7[Mn]+1.7[Co]+2.5[Fe].  [Equation 1]
 5. The titanium alloy of claim 1, wherein an elongation percentage of the titanium alloy is greater than or equal to 6%.
 6. The titanium alloy of claim 1, wherein the tensile strength of the titanium alloy is greater than or equal to 1,400 MPa.
 7. A rod material manufactured by using the titanium alloy described in claim
 1. 8. A plate material manufactured by using the titanium alloy described in claim
 1. 9. A rectangular lumber material manufactured by using the titanium alloy described in claim
 1. 10. A spring material manufactured by using the titanium alloy described in claim
 1. 